Personal interviewing, where the source of information is people, and the information is obtained verbally.
Postal questionnaires.
Observations or inspection. This method entails observing events as they happen and obtaining data by physically counting or measuring.
Abstraction, where the sources of data are documents.
Opinion surveys: an assessment of how a person or group feels about a particular issue.
Delphi technique: a method of the survey the requires surveying the same group of respondents repeatedly on the same issue in order to reach a consensus.
Self-ratings: a method used by participants to rank their own performance, knowledge and attitudes.
Case studies: experiences of selected persons/issues/items. An example can be to study the wear and tear on a machine.